Coherent vortex-dominated structures in the wake are ubiquitous in natural and engineered flows. The well-known 'von Karman street', in which two rows of counter-rotating vortices develop on the leeward side of a solid body immersed in a fluid, is only one such vortex-based structure in the wake. Recent work on fluid-structure interaction has shown that several other types of vortex structures can arise in natural and engineered systems. The production of these vortex structures downstream often mark the onset of qualitative and/or quantitative changes in the forces exerted on the vortex-shedding body upstream, and can be used as diagnostic tools for engineering structures undergoing Vortex-Induced Vibrations. This dissertation presents a two-part study of vortex dynamics in the laminar wakes of bluff bodies. The first part consists of a series of experiments on a transversely oscillating circular cylinder in a uniform flow field at Re≲250. These experiments were carried out in a gravity-driven soap film channel, which provides a `two-dimensional laboratory' for hydrodynamics experiments under certain conditions. In these experiments, we generated a `map' of the vortex patterns that arise in the wake as a function of the (nondimensional) frequency and amplitude of the cylinder's motion. Our results show that the '2P mode' of vortex shedding can robustly occur in the two-dimensional wake of an oscillating cylinder, contrary to what has been reported in the literature. By making small changes to the meniscus region of the soap film, we have explored possible mechanisms that can explain why the `P+S mode' of vortex shedding is usually reported to be more prevalent than the '2P mode' at low Reynolds number, when the flow is two-dimensional. In doing so, we have found that small modifications to the cylinder on the order of the boundary layer thickness can make a significant difference to the vortex shedding process. In the second part, we develop a generalized form of von Karman's drag law for N-vortex streets: periodic wakes in which the vortices are arranged in regularly-repeating patterns with N>2 vortices per period. The original form of von Karman's drag law then reduces to a special case of this generalized form, which has the potential to model several kinds of vortex-dominated wakes that have been reported in the literature. In this work, we show how this generalized drag law can be used to model '2P' and 'P+S' wakes in both `drag' and `thrust' form. As a contribution to the study of three-dimensional wakes, we also studied a periodic array of vortex rings, which are often used to represent the wakes of marine organisms like jellyfish and squid. We described the problem mathematically using a newly-developed Green's function, and comprehensively examine the fluid physics of such an array of vortex rings as a function of the non-dimensional parameters that govern this phenomenon. In the process, we have discovered a new type of topology that arises in this flow, which may have connections with the `optimal vortex formation length' of vortex rings. / Doctor of Philosophy / The interaction of solid objects with fluids such as water and air, often termed Fluid-Structure Interaction (FSI), gives rise to a wide variety of natural phenomena. Understanding FSI is important as an avenue of scientific interest as well as for engineering applications. In this dissertation, we are interested in the subset of FSI phenomena known as wakes: the fluid flow that is left behind when a solid moves rapidly through quiescent fluid, or when water or air flows rapidly past a stationary obstacle. In such situations, the flow is often rapidly rotating, taking the form of vortices or eddies, i.e., concentrated regions of rotating fluid. These eddies, or vortices, can be described mathematically using simple differential equations, and are the subject of the field of vortex dynamics, which is a branch of fluid mechanics. In the first part of this thesis, we have made contributions to the experimental study of FSI and wakes by making use of an experimental technique known as a gravity-driven soap film channel. In these experiments, a 'soap film', i.e., the surface of a soap bubble, is stretched out over a longitudinal channel formed by nylon wires and held taut in a rectangular shape. This rectangular film of soap is only a few micrometers thick, and is continuously fed by soap solution from the top and drained at the bottom, resulting in a steadily-flowing 'channel' of two-dimensional flow. In this experimental setup, we introduce a circular acrylic cylinder to serve as the archetypal 'obstacle' to fluid flow and oscillate it at a range of frequencies and amplitudes while using a high-speed camera to visualize the flow. This gives rise to a fascinating set of qualitatively distinct vortex patterns in the wake, with the structure depending on the selected frequency and amplitude of cylinder oscillation. In the second part of this thesis, we have developed mathematical models of two-dimensional wakes using a system of point vortices and of three-dimensional wakes using a system of circular vortex rings. We show how these idealized mathematical models of rotating flow, i.e., point vortices and vortex rings, can be used as building blocks for physically-plausible models of actually-occurring wakes, including those which were observed in the first part of this work. For two-dimensional wakes, we use Newton's laws applied to a fluid to determine the forces being exerted on a solid body, immersed in a fluid, whose wake takes the form of regularly-repeating vortices known as 'vortex streets'. This allows us to give, for the first time, theoretical predictions of the drag or thrust force associated with vortex streets such as those observed in our experiments.

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We examined the formation of Intrinsically Localized Modes (ILMs) for a pair of harmonic phonons along the direction [111] of the Sodium Iodide (NaI) crystalline lattice. The tendency for ILMs to form at a certain center-of-mass momentum ▁q and corresponding relative momentum vector ▁k is attributed to the van-Hove singularities condition in the non-interaction two-phonon density of states continuum. We observed that, as ▁q converges to the high-symmetry point L=▁q (π/a,π/a,π/a) of the Brillouin zone, the relative momentum vector ▁k remains invariant at ▁k (π/2,π/2,π/2) for a certain threshold value of ▁q, and coalesces at the upper-edge of the two-phonon density of states spectrum with high degeneracy in the two-phonon critical energy. We conclude that the excitation spectra of the pairs of harmonic phonon excitations become energetically degenerate past the threshold ▁q value towards L at the invariant vector ▁k, announcing the strong presence of ILMs. The calculated ILMs were observed at critical energies of 20.0 meV and 25.0 meV for the spring coupling constants ratios K_2/K_1 ≈0.598 and K_2/K_1 ≈0.202 respectively. Reports of Inelastic Neutron Scattering experiments have identified one-phonon breather excitations energy of 10.2 meV at elevated temperatures of 555 K. The formation of ILMs, or multi-phonon bound states, is expected to arise as a result of the anharmonic interactions that lift these degeneracies to enhance the formation of ILMs. / Physics

The magnetic properties of ultrathin magnetic films can be investigated in situ by the temperature dependent magnetic ac susceptibility x(T) using an optical technique - the surface magneto-optic Kerr effect (SMOKE). The performance of the ac susceptibility measurements depends primarily on the optical setup used to detect the Kerr effect and on the mechanical stability of the system. Modifications to the optical setup and the sample holder have significantly reduced the influence of noise due to mechanical vibrations. It has been found that the signal-to-noise ratio has been improved by at least a factor of 2.5 with respect to the previous setup, giving a detection limit of 15 nrad/Oe. This improvement makes measurements on antiferromagnetic ultrathin films feasible. Their susceptibility response has been estimated to be around 20-30 nrad/Oe. As a test study for the performance of the improved setup, transverse susceptibility measurements on 2 ML Fe/W(110) ferromagnetic ultrathin films are presented. These transverse susceptibility signals show interesting features. They have a narrow linewidth and are larger than expected from anisotropy considerations and other work. Also, it has been found that the in-plane and out-of-plane transverse susceptibilities arise from different mechanisms. Several scenarios that might explain the origin, size and shape of the observed signals are discussed. / Thesis / Master of Science (MSc)

<p> An extensive experimental study is made of the general behaviour of the impact dampers, using a mechanical model. Coefficient of restitution, Mass-ratio, and Gap-factor are the parameters which were changed during the course of investigation and their effects were observed. </p> <p> The noise level has been eliminated successfully. Dampers containing two particles in a single container are compared with single particles dampers and the latter are found to be relatively efficient. Results with the mass particle oscillating in the container filled with fluid indicate that friction forces acting on the mass-particle are detrimental to the efficiency of the damper. </p> / Thesis / Master of Engineering (ME)

La mesure des vibrations de la roue d'une turbine hydraulique en opération demeure encore aujourd'hui un dé, particulièrement à l'échelle modèle. L'installation d'appareils de mesure(jauges de contrainte et accéléromètres) sur la roue en rotation a un effet non négligeable sur l 'écoulement du fluide et affecte le comportement structural de la roue (masse ajoutée). Face à ce problème, l'utilisation d'appareils de mesure sans contact localisés sur le repère fixe est une solution intéressante pour mesurer les vibrations d'une structure en rotation. Ce mémoire de maîtrise présente une étude sur l'utilisation de sondes à courant de Foucault localisées sur le repère fixe pour mesurer les vibrations au niveau de la ceinture d'une roue de type Francis en opération. D'abord, une méthode de mesure est développée an d'éliminer le dédoublement des résonances présent dans la réponse en fréquence des mesures de vibrations d'une structure en rotation prises à partir du repère xe. Une validation numérique de cette méthode démontre qu'il est possible de réaliser l'analyse modale de la structure en rotation. Par la suite, un montage expérimental est conçu pour mesurer les vibrations d'un anneau en rotation et immergé sous l'eau an d'appliquer la méthode proposée. Finalement, une campagne de mesures préliminaires est réalisée avec le montage expérimental an de tester les sondes à courant de Foucault pour mesurer les vibrations dans des conditions opératoires similaires à celles retrouvées dans les turbines hydrauliques. Ces mesures préliminaires permettent également d'observer le comportement du montage conçu sur les mesures vibratoires de l'anneau. / The vibrations measurement of hydraulic turbine runners in operation is still actually a challenge, especially at the model scale. It is possible to x sensors on the rotating part likes train gauges and accelerometers, but these sensors have a significant effect on the structural behavior of the runner (added mass) and on the hydraulic ow. To deal with this issue, the usage of a non-contact measuring devices located on the non-rotating frame is an interesting solution to measure the vibrations of a rotating structure. This Master's thesis presents the study of the use of eddy current sensors located on the fixed frame to measure the vibrations of an operating Francis runner's band. First, a measurement method is developed to eliminate the duplication of resonances in the frequency response of the vibration measurements of a rotating structure taken from the non-rotating frame. A numerical simulation of this method shows that it is possible to perform the modal analysis of the rotating structure. Subsequently,an experimental bench test is designed to measure the vibrations of a submerged rotating ring in order to apply the proposed method. Finally, a set of preliminary measurements is achieved with the experimental bench to test the eddy current sensors to measure the vibrations under similar operating conditions that can be found in the hydraulic turbines. These preliminary measurements allow us to observe the behavior of the bench test on the vibration measurements of the ring.